Thermal batteries are a well-known type of primary reserve battery that have a long shelf life at ordinary temperatures and are actuated by heating to a temperature that melts a salt electrolyte. The batteries are useful in applications for a high energy output over a short time. The batteries comprise a plurality of cells, each of which includes an anode, a cathode, an electrolyte and a heat source, usually an ignitable exothermically reactive chemical charge or pyrotechnic. A variety of electrochemical systems are known for use in thermal cells.
The anode is an electropositive metal, commonly calcium or lithium alloys and more recently lithium as described in U.S. Pat. No. 3,930,888, U.S. Pat. No. 3,891,460 and U.S. Pat. No. 4,221,849. The electrochemical reactant of the cathodes, generally referred to as depolarizers, are electropositive materials that are reduced in the electrochemical cell reaction and include phosphates, borates, metal oxides and chromates. Calcium chromate and vanadium pentoxide have been widely used depolarizers as, more recently, has iron pyrite (FeS.sub.2), described in U.S. Pat. No. 4,119,769. Under current practice powdered depolarizer is mixed with electrolyte and binder and compacted to form a homogeneous wafer cathode, which may be a layer of a composite wafer also having an electrolyte layer made up of electrolyte and binder. The electrolytes are ordinarily mixtures of alkali metal halides, most often a eutectic mixture of lithium chloride and potassium chloride. The state of the art thermal battery comprises a lithium anode immobilized with iron powder, a LiCl-KCl eutectic electrolyte immobilized with refractory powder binder, and an FeS.sub.2 cathode with electrolyte and binder. A pyrotechnic heat source wafer of iron powder and potassium perchlorate is interposed between each cell.